Polypropylene homo- and random copolymers are inherently translucent due to the nature of the polymer morphology. Clarity improvement can be realized by controlling the rate of crystal growth as the molten polymer solidifies upon cooling in the final molding step. This requires the use of so-called "nucleating agents" which act to reduce spherulite size by shifting the nucleus formation curve closer toward the polymer melting point, thereby making nucleation rather than spherulite growth the dominant solidification process. This mechanism results in enhanced clarity in molded articles of propylene homo- and random copolymer. In general, the nucleating agents used are salts of aromatic or alicyclic carboxylic acids (e.g. sodium benzoate), which are very effective in propylene polymers produced in the presence of non-supported conventional catalysts of the type n.TiCl.sub.3.AlCl promoted with organoaluminum catalysts.
U.S. Pat. No. 3,299,029 discloses a method of nucleating propylene polymers using as the nucleating agent an aluminum salt of a carboxylic acid such as benzoic acid, adipic acid, etc., which salt has been formed by reacting the carboxylic acid and a basic aluminum compound such as aluminum hydroxide or aluminum alkoxide in the presence of the polymer. The carboxylic acid and the aluminum compound are mixed in the form of separate suspensions with a suspension of the polymer and the resultant mixture is then freed from liquid prior to the final crystallization step. An obvious disadvantage of such a process is the cost associated with the liquid removal step.
Recently new catalysts have been developed which are far more active than the aforementioned conventional catalysts in the polymerization of 1-olefins. Briefly described, these catalysts are comprised of a titanium halide catalyst component supported on magnesium dihalide and an alkylaluminum compound, which can be present as a complex with an electron donor compound. These catalyst compounds have been described in the patent literature, e.g. in U.S. Pat. Nos. 3,830,878, 3,953,414, 4,051,313, 4,115,319 and many others.
The productivities obtained with these new catalysts are extremely high, resulting in polymers containing such small quantities of acidic residual catalyst that the conventional deashing step can be dispensed with. Using conventional nucleating agents such as sodium benzoate in propylene polymers produced with the newer, high-activity support-based coordination catalysts have failed to result in acceptable improvement in clarity, because of the presence of the aforementioned catalytic residues and neutralizing agents in the polymer.
It is, therefore, an object of the present invention to provide a novel process for the production of propylene polymer composition of improved clarity, which polymer has been prepared in the presence of support-based high-activity coordination catalyst.